A single-electron device, as the term is generally understood, is a device which controls the movement of individual electrons in solids. They commonly take the form of tunnel junctions, consisting of two conductors separated by an ultrathin (less than 100 nanometers) layer of insulating material. When a voltage is applied across the tunnel junction, inducing current (electron movement) to flow in the conductors, a surface charge accumulates on one surface of the conductor against the insulating layer, with an equal and opposite charge on the other insulator/conductor surface. When this surface charge exceeds a predetermined value, an electron tunnels through the insulating layer, thereby reducing the surface charge. When the surface charge is less than the predetermined value, tunnelling is suppressed (the Coulomb blockade). As a surface charge increases due to continued current flow in the conductor, i.e. continued application of the voltage across the insulator, the charge increases again, until the Coulomb blockade is overcome, whereupon another single electron tunnels through. The phenomenon can be observed experimentally, as a fundamental relation between applied voltage and the frequency of oscillation in the current flowing through the junction. The height of the Coulomb blockade depends upon the conductance and capacitance of the insulating layer.
A general description of single electronics and single-electron devices and their method of operation appears in "Scientific American", June 1992, page 80, in an article by Likharev and Claeson.
These single-electron phenomena show promise for application to digital integrated circuits. Currently computer chips can have a density of about ten million devices per square centimeter, to handle the commonly used digital electrical pulses. Using single electronics, even further reductions in size are possible, since bits of information can be represented through the passage of individual electrons. For this to happen, however, techniques need to be developed to fabricate complex structures whose smallest dimension is, controllably and reproducibly, less than 100 nanometers. Such techniques also need to be capable of operation economically and efficiently, for the production of single-electronic devices incorporating one or more such tunnel junctions.